This invention relates to transfer printing, and more particularly to electrostatic transfer printing.
Various types of electrostatic transfer printers can be found in the prior art. Examples are F. A. Schwertz U.S. Pat. No. 3,023,731; Richmond Perley U.S. Pat. No. 3,701,996; and T. Doi et al., U.S. Reissue Pat. No. 28,693. Electrostatic transfer printers may be classified generally according to the way in which the latent electrostatic image is formed. One prior art approach utilizes metal styli at minute distances from the surface of the dielectric transfer drum. The styli are electrically pulsed to provide a latent electrostatic image by air gap breakdown. This technique has the disadvantage of not allowing for multiplexing of the charging styli. In addition, the necessity for maintaining a very small air gap breakdown distance requires extremely close tolerances which limit the practicability of this technique.
Air gap breakdown, i.e. discharges occuring in small gaps between electrodes, or between a conductive surface and the surface of a dielectric material, are widely employed in the formulation of electrostatic images. Representative U.S. Pat. Nos. are G. R. Mott 3,208,076; R. F. Howell 3,438,053; E. W. Marshall 3,631,509; A. D. Brown, Jr. 3,662,396; R. T. Lamb 3,725,950; A. E. Bliss et al. 3,792,495; G. Krekow et al. 3,877,038; R. F. Borelli 3,958,251; and Terazawa 4,096,489.
Another type of electrostatic printer found in the prior art employs an ion source in the form of a corona point or wire used together with an image defining mask. U.S. Pat. No. 3,863,261 to Klein illustrates this type of ion generating apparatus. Because of the inherently low current densities available from traditional corona discharges, this method is impractical for high speed printing. The use of coronas also poses significant difficulties in maintenance. Corona wires are fragile, and because of their high operating potentials, tend to collect dirt and dust. Hence they must be frequently cleaned or replaced.
Corona discharge devices which enjoy certain advantages over standard corona apparatus are disclosed in Sarid et al., U.S. Pat. Nos. 4,057,723; Wheeler et al. 4,068,284; and Sarid 4,110,614. These patents disclose various corona charging devices characterized by a conductive wire coated with a relatively thick dielectric material, in contact with or closely spaced from a further conductive member. A supply of positive and negative ions is generated in the air space surrounding the coated wire, and ions of a particular polarity are extracted by a direct current potential applied between the further conductive member and a counterelectrode. Such apparatus overcomes many of the above-mentioned disadvantages of prior art corona charge and discharging devices but is unsuitable for electrostatic imaging. This limitation is inherent in the feature of large area charging, which does not permit formation of discrete, well-defined electrostatic images.
Furthermore, these devices are characteristically maintained at greater distances from the member to be charged or discharged than is characteristic of the imaging device of the present invention, and hence require substantially greater extraction potentials. Another approach to electrostatic transfer printing focuses on the method by which the toned image is transferred and fused onto the receptive sheet. The transfer printing system of R. Perley, U.S. Pat. No. 3,701,996, involves simultaneous transfer and pressure fusing by passing a receptive sheet between the transfer and pressure drums. This patent does not contain sufficient teachings of suitable roller materials and characteristics to enable the skilled artisan to make or use such a printer. The Perley printer creates the latent electrostatic image using corona styli, which imposes limitations on image quality and speed of operation. In P. Pederson, U.S. Pat. No. 3,874,894, a nylon-six sleeve is provided on at least one of a pair of pressure rolls, but the drums are used only for fixing the already transferred toner, an arrangement which adds significant complexity to the overall system. Brenneman et al. U.S. Pat. No. 3,854,975, discloses pressure fixing apparatus involving a pair of compliant rollers, or a compliant roller and a relatively rigid roller; again, such apparatus is used only to fuse a previously transferred toner image.
Accordingly, it is an object of the invention to facilitate electrostatic transfer printing. A related object is to reduce critical mechanical tolerances in providing a latent electrostatic image. Another related object is to reduce the maintenance problems associated with the formation of such an image.
A further object of the invention is to achieve increased electrostatic printing speed. A related object is to do so by using a reliable, easily controlled ion source. A still further object is to achieve relatively uniform charge images which may be toned with good definition and dot fill. A further related object is to provide a matrix selection (or multiplexed) method of dot matrix printing.
Another object of the invention is to achieve an image-bearing member with surface resistivity sufficient to prevent image degradation from the time when the image is presented to the surface until the image is toned. Still another object is to utilize a surface with high abrasion resistance, and sufficient smoothness to provide complete transfer of toner to a receptor sheet. A still further object is to realize a transfer surface not subject to significant distortion.
Yet another object is to facilitate the erasure of latent residual electrostatic images. A related object is to avoid ghost images in subsequent printing cycles.